In a typical exposure apparatus of this type, a light beam emitted from a light source is entered into an optical integrator such as a fly-eye lens, and then forms a secondary light source which is composed of a plurality of light sources on a rear focal point of the optical integrator. The light beam from the secondary light source is condensed by a condenser lens and forms an illumination field on a predetermined position which conjugates with a mask. A mask blind which functions as an illumination field stop is provided in the vicinity of the illumination field.
The light beam from the illumination field on the predetermined position is restricted by the illumination field stop and illuminates the mask in which a predetermined pattern is formed thereon by superimposing the light onto the mask through a relay imaging optical system. As a result, an image of an opening portion of the illumination field stop is formed on the mask as an illumination area. The light which passes through the pattern of the mask is imaged onto a photosensitive substance through a projection optical system. Consequently, projection exposure (transference) of the pattern of the mask onto the photosensitive substance is performed.
In the above typical exposure apparatus, in order that the effects of gravity on the mask and photosensitive substance act symmetrically around an optical axis of the projection optical system toward the mask and photosensitive substrate, a structure is commonly employed in which the optical axis of the projection optical system is made to coincide with the direction of gravity (vertical direction) and the mask and the photosensitive substrate are held in horizontal direction. In this structure, the relay imaging optical system which optically conjugates the illumination field stop and mask is provided on the top portion of the exposure apparatus. Therefore, it is preferable that the relay imaging optical system be miniaturized and reduced in weight in order to obtain the exposure system having a structure which is resistant to the effects of vibration. In other words, it is preferable that the relay imaging optical system be composed of a small number of lenses.
Furthermore, in recent years, an exposure light tends to have a short wavelength in accordance with the development of miniaturization of the patterns to be transferred, and a KrF excimer laser having wavelength of 248 nm or an ArF excimer laser having a wavelength of 193 nm is now being used as the light source of the exposure light. In this case, phenomena that surfaces of the lenses may easily be clouded and permeability of the lenses may easily be decreased due to detrimental gases are known. Therefore, the relay imaging optical system which is composed of a small number of lenses is advantageous to avoid these phenomena.
The present invention was provided in consideration with the foregoing problems, and one object of the invention is to provide a relay imaging optical system which is composed of a small number of lenses and can be miniaturized and reduced in weight with maintaining required optical characteristics. Furthermore, the other object of the invention is to provide an illumination optical apparatus and an exposure apparatus having a relay imaging optical system which is composed of a small number of lenses and is miniaturized and reduced in weight.